Index: lib/Transforms/InstCombine/InstCombineCasts.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -1083,6 +1083,18 @@
     uint32_t SrcBitsKept = SrcTy->getScalarSizeInBits()-BitsToClear;
     uint32_t DestBitSize = DestTy->getScalarSizeInBits();
 
+    // Preserve any debug values referring to the zext by emitting
+    // a framgent expression.
+    insertReplacementDbgValues(
+        *Src, *Res, *std::next(CI.getIterator()),
+        [&](DbgInfoIntrinsic &OldDII) -> DIExpression * {
+          auto DIE = DIExpression::createFragmentExpression(
+              OldDII.getExpression(), SrcBitsKept, DestBitSize);
+          if (DIE == None)
+            return nullptr;
+          return DIE.getValue();
+        });
+
     // If the high bits are already filled with zeros, just replace this
     // cast with the result.
     if (MaskedValueIsZero(Res,
Index: test/Transforms/InstCombine/debuginfo-trunc-and-zext.ll
===================================================================
--- /dev/null
+++ test/Transforms/InstCombine/debuginfo-trunc-and-zext.ll
@@ -0,0 +1,28 @@
+; RUN: opt -debugify -instcombine -S < %s | FileCheck %s
+
+target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32"
+
+; Test that when merging zext we preserve
+; the debug information via fragments in vector
+; and non-vector cases
+
+define <2 x i64> @test-vector(<2 x i64> %A) {
+  %trunc = trunc <2 x i64> %A to <2 x i32>
+  %and = and <2 x i32> %trunc, <i32 23, i32 42>
+  %zext = zext <2 x i32> %and to <2 x i64>
+  ret <2 x i64> %zext
+}
+
+; CHECK-LABEL: define <2 x i64> @test-non-vector
+; CHECK: call void @llvm.dbg.value{{.*}}
+; CHECK: call void @llvm.dbg.value({{.*}}, metadata !DIExpression(DW_OP_LLVM_fragment, {{.*}}
+
+define i32 @test-non-vector(i32 %x, i32 %y) {
+  %A = trunc i32 %x to i8
+  %B = trunc i32 %y to i8
+  %C = mul i8 %A, %B
+  %D = zext i8 %C to i32
+  ret i32 %D
+}
+
+; todo: add checks for non-vector